1. Field of the Invention
The invention relates generally to wireless communication and more particularly to systems and methods for controlling the output power in a wireless communication device.
2. Background
There are several factors that impact the transmit power level in the transmitter of a wireless communication device. Two factors that limit the transmit power level, for example, are: 1) Specific Absorption Rate (SAR) requirements; and 2) Adjacent Channel Power Ratio (ACPR) requirements. SAR is a metric used to evaluate compliance of portable devices with the maximum permissible exposure limits as defined in the FCC guidelines on human exposure to Radio Frequency (RF) emissions. Effectively, the FCC guidelines place a limit on the maximum transmit power of a communication device in order to prevent exposure by users of such devices to excessive levels of RF energy.
ACPR is generally defined as the ratio of the average power in the adjacent frequency channel to the average power in the transmitted frequency channel. In other words, a wireless communication device is configured to transmit over a specific frequency channel at any given time. But due to inherent linearity and other limitations of the components that comprise a communication device transmitter, it is very difficult to prevent the energy transmitted by the device from spreading over into adjacent channels. If too much energy resides in the adjacent channels, then it can interfere with devices operating on those channels. Therefore, many wireless communication standards define limits for ACPR, and even when the applicable standard does not define a limit, ACPR is still a practical limitation.
In order to maintain acceptable SAR and ACPR limits, conventional communication device transmitters typically comprise a power detector, to detect the transmit power level, and an isolator to isolate the transmitter from reflected energy generated at the interface between the transmitter and the device""s antenna. For example, in a Frequency Modulation (FM) transmitter, SAR is the limiting issue. Therefore, a power detector can be used to ensure that the output power of the transmitter does not exceed the FCC specified limits. In a transmitter that is implementing a complex modulation scheme, such as Code Division Multiple Access (CDMA) or Time Division Multiple Access (TDMA), on the other hand, there are much more stringent linearity requirements. Thus, ACPR is the limiting issue, although SAR still applies. If the transmitter attempts to produce too much or excessive power is reflected back from the antenna into the transmitter, the linearity and, therefore, the ACPR can be substantially degraded. Accordingly, conventional devices often insert an isolator to block the reflected power and have some means to limit the maximum RF output power if there is a danger of exceeding the transmitter rating before reaching the SAR threshold
While the conventional detector/isolator approach has certain advantages, it also has certain limitations that can substantially impact the performance of a wireless communication device. For example, the impedance of the transmission line that conveys the transmitted power to the antenna is designed to match the impedance of the antenna in order to reduce the amount of reflected energy and increase transmission efficiency. But when the communication device is placed next to the human head, for example, the impedance of the antenna changes due to the proximity with the head. As a result, more power is reflected back toward the transmitter. When this reflected energy reaches the isolator it is dissipated as heat. Therefore, the resulting radiated transmit power is much lower than it otherwise could be, even taking into account the SAR limitation.
Additionally, the isolator introduces extra loss into the transmission path that is typically on the order of 0.5 dB. Therefore, the transmitter must supply an extra 0.5 dB of power in order to compensate for the extra loss. Increasing the power, however, also increases the ACPR, i.e., increases the amount of energy in the adjacent channels. Because ACPR is predominantly a 3rd order product, the resulting increase in ACPR is approximately 3 times the increase in transmit power, or 1.5 dB, which can lead to noncompliance with the ACPR requirements. Thus, as can be seen, the conventional detector/isolator approach can have a substantial negative impact on the performance of a wireless communication device.
FIG. 1 illustrates an exemplary wireless communication transceiver 100. Such a transceiver can be included in a wireless communication device, thus enabling the device to communicate over a wireless communication channel 124 in a wireless communication system. Transceiver 100 actually comprises a receive path 106 and a transmit path 110. Preferably, both paths are interfaced with antenna 102 via a duplexer 108. Duplexer 108 essentially acts as a filter that is configured to shunt incoming RF signals received by antenna 102 to receive path 106. Duplexer 108 is further configured to send outgoing RF signals from transmit path 110 to antenna 102, while providing isolation between paths 106 and 110 so that the incoming and outgoing signals do not interfere with each other.
The received RF signals are then demodulated and processed so as to extract a baseband information signal in the receive portion of transceiver 100 (not shown). Preferably, the baseband information signal is then decoded and processed in a baseband processor (not shown), such as a Mobile Station Modem (MSM). The MSM, or equivalent, is also preferably responsible for generating and encoding baseband information signals that are to be transmitted over communication channel 124. The baseband information signals generated by the MSM (not shown) are then modulated with a RF carrier in the transmit portion of transceiver 100, which generates a RF transmit signal to be transmitted via antenna 102.
The transmit portion of transceiver 100 is also preferably configured to set the power level of the RF transmit signal. In general, Power Amplifier (PA) 120 in conjunction with Variable Gain Amplifier (VGA) 122 generate the required power level as demanded by the MSM. PAs are typically key components in any high frequency RF transmitter design. This is because RF transmitters typically require high output power to compensate for path losses in communication channel 124 and to ensure an adequate signal strength at the base station associated with channel 124. Since the base station can be miles away, it can be difficult to achieve adequate receive power at the base station. At the same time, if the signal power at the base station is too high, then it may interfere with reception by the base station of transmit signals from other devices within the communication system. Transmitting at higher power levels also reduces battery operating time. Therefore, while it is important to ensure an adequate transmit power level, it is also important to ensure that the transmit power level is not too high. Thus, power control in a wireless communication device is an important aspect of wireless communication.
In conventional wireless communication systems, power control is often performed in the wireless communication device. For example, the base station can be configured to measure the power level of a received transmit signal and determine if it is too high or too low. The base station can then be configured to transmit commands to the wireless communication device instructing the device to turn its power up or down. CDMA communication systems, for example, use such a power control loop. In a CDMA system, the goal of the base stations is to receive signals from each of the devices with which it is communicating at the same receive power level. In fact, such power equalization at the base station for each of the devices in communication with the base station is a critical aspect of CDMA operation. Thus, power control is a critical component of device operation in a CDMA system, although it is similarly important in many types of wireless communication systems.
For illustrative purposes, a simplified power control loop operation for a CDMA system is described in the following paragraphs in conjunction with the flow chart of FIG. 2. The process of FIG. 2 is intended to illustrate the need for power control and the role it plays in wireless communication. It should not, however, be seen as limiting the systems and methods described herein to any particular type of power control, or any particular power control approach. Nor should it be seen as limiting the systems and methods described herein to any particular type of wireless communication system.
Again, in a CDMA system, such as an IS-95 compliant system, the transmit power is controlled in the communication device so that devices communicating with the same base station appear to have the same signal strength at the base station. In each device, the transmit power is variable to compensate for changes in the signal strength as received by the base station. The signal strength at the base station can vary due to changing distances between a communication device and the base station and such factors effecting communication channel 124 as multipath fading, varying terrain topology, and varying environmental conditions.
Referring to FIG. 2, the power control loop in a CDMA system, begins by entering an open loop power control sequence 234 in step 202. Once in open loop sequence 234, the device will estimate an initial transmit power in step 204. For example, the initial estimate can be made using a predetermined loop power equation such as the following equation:
Rx power+Tx power=xe2x88x9273 dBmxe2x80x83xe2x80x83(1) 
In equation (1), Rx power is the signal strength of a RF signal received from the bases station over communication channel 124 by antenna 102. Once this received power level is determined, e.g., via a Received Signal Strength Indication (RSSI) measurement, then it can be used by loop equation (1) to determine the initial transmit power, or Tx power, in step 204. The device will then transmit a signal at this initial power level to the base station in step 206 and wait for an acknowledgement from the base station in step 210. If the device does not receive an acknowledgement in step 212, then it will increase the transmit power in step 214, transmit again in step 216, and again wait for acknowledgement (step 210). Typically, a device may need to increase its power 1 or 2 times before receiving the acknowledgement.
The open loop process is a coarse estimate of the required transmit power. Thus a tolerance of +/xe2x88x929 db is, for example, allowed on the open loop estimate of the required power. Once the device receives an acknowledgement (step 212), however, it enters, in step 218, a closed loop power sequence 236 in which the transmit power level estimate is refined. The goal of closed loop power control sequence 236 is to ensure that the power received at the base station is the minimum level of power required for each device with which the base station is communicating.
Once in closed loop sequence 236, the base station measures the received power-to-interference-ratio (Eb/Io) and compares the measured value to a minimum and a maximum threshold in step 222. If the base station determines that the measured Eb/Io is above the maximum threshold in step 224, then, in step 226, it sends a command to the device to reduce its power. If, on the other hand, the base station determines in step 228 that the Eb/Io is below the minimum threshold, then it sends a command to the device to raise its transmit power level in step 230. Of course, the measured Eb/Io may be between the minimum and maximum thresholds in which case there would be no need for the device to modify its power. In such a situation, the device can be instructed to maintain the same transmit power level in step 232. The measurement (step 220) and comparison (222) is preferably repeated periodically, e.g., every 1.25 ms, or 800 times per second. Thus, it can be seen that power control plays an important role in proper operation of a communication device within a wireless communication system.
Referring again to FIG. 1, there are several ways that a device can control the output power in transceiver 100. Because the transmit power may have to be varied over a large range typically in excess of 70 dB, one way to control the output power is by varying the gain of VGA 122. Further to ensure improved transmitter efficiency at lower power levels, PA bias 126 may also be adjusted as required. VGA 122 can be configured to amplify the transmit signal before it is sent to PA 120. How much VGA 122 amplifies the transmit signal can be controlled via a TX POWER CONTROL signal 128, which can be generated by an MSM (not shown) or some other baseband control circuit (not shown).
Proper control of the transmit power level, as explained above, can be critical for efficient operation of a wireless communication device in a wireless communication system. There are other limits, however, on the transmit power level in transceiver 100. For example, as explained above, SAR limitations may restrict the transmit power level regardless of what the power control loop operation may dictate. To ensure that the SAR limitations are not exceeded, conventional wireless communication devices typically employ some sort of power detector 114. In transceiver 100, power detector 114 comprises a diode 118. The output 130 of detector 114 is then sent to a MSM (not shown) or some other baseband control circuit (not shown).
The analog voltage generated by the detector 114 represents the generated transmit power level and can be converted to a digital number, by means of an analog-to-digital converter for example, such that the MSM (not shown) can adjust the gain of VGA 122 accordingly to meet the desired transmit power level. Notably, however, such a power detection scheme does not take into account reflected power that is dissipated in isolator 112. Isolator 112 is included because the reflected power can have an adverse effect, e.g., increased ACPR especially at high transmit power levels, if it is allowed to interact with the transmit signal being generated by PA 120.
Reflected power occurs because of mismatches in the impedance between the transmission line 132 conveying the transmit signal and antenna 102. The amount of reflected power is determined by the reflection coefficient, which is a measure of the mismatch in impedance between antenna 102 and transmission line 132. To lower the reflection coefficient, and thereby reduce the amount of reflected power, conventional devices typically include matching circuit 104. The purpose of matching circuit 104 is to match the impedance of transmission line 132 with that of antenna 102. In practice, however, it is very difficult to achieve a perfect match. Therefore, some of the transmit power is reflected back toward PA 120. The reflected power generates a standing wave on transmission line 132 from the interaction between the forward and reflected signals. Voltage Standing Wave Ratio (VSWR) is a metric used to determine how much of the transmitted power is making it out at antenna 102. VSWR can be defined by the following equation:
VSWR=(Vfwd+Vref)/(Vfwdxe2x88x92Vref)xe2x80x83xe2x80x83(2) 
In equation (2), Vfwd is a measure of the voltage level of the transmit signal on transmission line 132 and Vref is a measure of the voltage level of the reflected signal. If impedance matching circuit 104 provides a perfect match, then the ratio is 1:1 and maximum power will be delivered to antenna 102. Any deviation from this condition, i.e., a VSWR greater than 1:1, and less than maximum power is delivered to antenna 102.
If it were not for isolator 112, the reflected power would travel back toward PA 120, reflect again, and travel back toward antenna 102. Therefore, at least some portion of the reflected power would eventually get out at antenna 102. Thus, a transceiver can be designed for a VSWR of approximately 2:1 and still have sufficient performance. But in transceiver 102, the reflected power is actually dissipated in isolator 112 as heat. Thus, any deviation from a VSWR of 1:1 results in wasted transmit power and reduced talk time. But detector 114 does not take into account the effect of isolator 112 and, as a result, transceiver 100 can actually be operating well below SAR limits when the device is limiting the PA output due to the measurements from detector 114.
For example, it is not uncommon for the VSWR to degrade from 2:1 to approximately 3:1 when a wireless communication device is placed next to a human head during operation. A VSWR of 3:1, however, means that 25% of the transmit power is reflected back into the device, where it is dissipated as heat in isolator 112. Because this power is wasted, the power level is much lower than expected. This not only results in inefficient operation of transceiver 100, but can actually cause the device to lose its connection with the base station. Even if detector 114 is not causing the transmit power level to be limited, PA 120 must operate at excessive power levels in order to compensate for the transmit power wasted in isolator 112, which not only reduces battery capacity but can also raise ACPR.
As mentioned above, isolator 112 also typically adds approximately 0.5 dB of loss to the transmission path, which requires PA 120 to increase the transmit power level to compensate. Not only does this result in inefficient operation of PA 120 and reduces battery life among other things, but it also causes the ACPR to increase. Because ACPR is a 3rd order product, a 0.5 dB increase in transmit power will result in approximately a 1.5 dB increase in ACPR, which may cause excessive interference in the adjacent channel.
In view of the above discussion, it can be seen that the use of detector 114 and isolator 112 can have a substantial negative impact on the performance of transceiver 100.
Thus, it is an objective of the invention to eliminate the need for an isolator in the transmitter of a wireless communication device. It is a further objective to provide dynamic control of the transmit power level in the transmitter in order to maintain maximum output power, while still meeting such limitations as SAR and ACPR. In one aspect of the invention, the systems and methods for controlling output power in a communication device use a power control circuit to sense both the forward and reflected power levels in the transmission path between the transmitter and the antenna. The power control circuit is then configured to control the transmit power level of the transmitter based on the power levels that it senses.
For example, in one particular implementation the transmitter power level can be optimally controlled by adjusting a Power Amplifier (PA) drive signal in accordance with inputs from a MSM, such as a demanded transmit power input, and by forward/reverse powers detected by the power control circuit. For example in an extreme case where forward and reflected powers are identical (100% power reflection), this invention would reduce drive power to the PA to a very low level and avoid the condition of dissipating excess power as heat. Further the power control system would adjust the PA bias to the most appropriate point for a given output level. For higher power levels where high linearity is required the bias would be increased. For lower power levels the bias point would be decreased, but not to a point where ACPR is impacted.
In another aspect of the invention, the power control circuit may also adjust an impedance of an impedance matching circuit based at least in part on the reflected power level sensed by the bi-directional coupler detector.
These embodiments as well as other features, advantages, and embodiments are described below in conjunction with the following drawings.